[ARC5] HF Converter - feeding mixer with a square wave?

[Tom Lee]

Hi Leslie,

An ideal mixer multiplies an rf input signal by a sinusoidal LO, and 
produces only sum and difference frequency components. It is a linear 
system in the important sense that the output is proportional to input, 
for a fixed LO amplitude. Many textbooks leave the unfortunate 
impression that mixers are nonlinear.

A four-diode ring mixer and a Gilbert-type mixer (more properly a Jones 
mixer, but if you say "Jones mixer" most people wouldn't know what you 
were talking about) usually operate with a large enough LO drive that is 
effectively multiplying the rf input by a square wave LO (even if the LO 
waveform is a sine). As you correctly surmise, multiplication by a 
square wave will produce a host of mixing terms beyond the desired one. 
Putting your trust in M. Fourier as you suggest, you can see that you 
will not only multiply the input by the LO's fundamental, but also by 
the LO's higher-order odd harmonics.

Filtering the output can eliminate those other mixing products, but that 
doesn't fully solve all problems. The mixer will happily down- (or up-) 
convert input signals that are displaced from the LO's odd harmonics by 
the desired IF. These image signals will therefore produce mixer outputs 
that interfere with those produced by the desired rf input. If your 
frequency plan is such that there are such image signals, you will have 
to filter those, too.

If you reduce the LO drive in an effort to approach the ideal of 
multiplication by a pure sine, you inevitably discover that the image 
problem gets replaced by a mixer noise problem. Mixer noise figures are 
quite a bit higher than those of amplifiers.

All of the foregoing assumes that you are employing a double-balanced 
architecture, so that problems of LO and rf feedthrough are suppressed. 
If a double-balanced topology isn't used, then you have additional 
problems to solve (most readily by switching to a double-balanced 
circuit). A common type of low-complexity mixer exploits the 
nonlinearity of a device (e.g., vacuum tube) to provide an incidental 
multiplication. Since multiplication is only a side effect in such 
devices, these types of mixers suffer from many defects. But they win 
where simplicity is prized over performance.

-- Cheers,
Tom

-- 
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/16/2022 18:23, Leslie Smith wrote:
>
>         HF converter - 80M to BC-453 tunable IF.
>
> I wish to build a simple solid state converter to bring the lower part 
> of 80m down to suit a BC-453 working as a tuneable IF.   The LO 
> frequency would therefore be 3200kHz.  Crystals are readily available 
> for 6400kHz.  Consequently I am considering dividing the crystal 
> frequency by 2 using a JK (or similar) digital divider.   This raises 
> a question.  Normally a mixer is driven with (something approximating) 
> a sine-wave - a signal with no harmonics.  A digital divider provides 
> a square wave, with many odd harmonics in the signal.  The amplitudes  
> of the harmonic fall progressively, so that the third has an amplitude 
> of one third the fundamental.  The fifth fifth - one fifth and so on.  
> You can check this easily using MS excel spreadsheet.  (or just trust 
> Mr. Fourier).
>
> Question:  How will the presence of these harmonics affect the 
> operation of the mixer.  Will a square wave produce a "family" of 
> intermodulation signals that I find undesirable?  Am I chasing the 
> wrong birdie up a tree?
>
> Leslie
>
> PS - 3200kHz crystals are available, but not so easily got as crystals 
> for 6400kHz, so the idea of using a sine-wave isn't out of the question.
>
>
>
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